By Louis Charbonneau

UNITED NATIONS (Reuters) - Russian Deputy Foreign Minister Sergei Ryabkov said on Tuesday he was "optimistic" about talks on a fourth round of U.N. sanctions against Iran between Russia and five other world powers.

"These talks are slowly moving forward," said Ryabkov, who is Russia's lead negotiator on the Iranian nuclear issue. "Definitely there is still some space to bridge over, but I wouldn't exaggerate or over-exaggerate the differences."

"I'm reasonably optimistic," said Ryabkov, who is in New York for the opening of a month-long conference taking stock of the 1970 nuclear Non-Proliferation Treaty.

U.N. diplomats from Russia, the United States, Britain, France, Germany and China have been meeting nearly every day for weeks to hammer out a draft sanctions resolution to submit to the full Security Council for a vote.

Russia and China, Western diplomats say, have been pushing the four Western powers to dilute some of the measures in the U.S.-drafted sanctions proposal. Moscow and Beijing have strong commercial ties with Iran.

He reiterated Russia's position that the point of a fourth round of U.N. sanctions should not be to punish Iran but to "strengthen the non-proliferation regime." Diplomats have said that this means Russia wants any new U.N. measures to focus exclusively on Iran's nuclear and missile industries.

Diplomats have told Reuters that the U.S. draft proposes new curbs on Iranian banking, a full arms embargo, tougher measures against Iranian shipping, moves against members of Iran's Islamic Revolutionary Guard Corps and firms they control, and a ban on new investments in Iran's energy sector.

U.S. President Barack Obama had originally wanted a sanctions resolution in place by the end of April, but diplomats say the talks could drag on until at least June.

The United States, European Union and other nations suspect Iran is developing the capability to produce nuclear weapons under cover of a civilian atomic energy program. Iran denies the charges and insists its program is entirely peaceful.

The 15-nation Security Council imposed three rounds of sanctions on the Islamic Republic in 2006, 2007 and 2008 for refusing to suspend its uranium enrichment program. Tehran says it will never give up what it says is a sovereign right guaranteed under the NPT.

(Reporting by Louis Charbonneau; Editing by Eric Beech)